Composite materials (often abbreviated as ‘composites’) are engineered materials made from two or more constituent materials that generally remain separate and distinct on a macroscopic level while forming a substantially unitary component. There are two general categories of constituent materials: matrix and reinforcement. At least one portion (i.e., mass fraction) of each type of material is typically employed. Matrix material generally surrounds and supports the reinforcement material by maintaining the disposition of reinforcement material relative to the matrix material. The reinforcement material generally operates to impart mechanical, electrical and/or physical properties to enhance the properties of the matrix material. A synergy between the matrix and reinforcement material is produced that is otherwise unavailable in homogeneous compositions. The most primitive composite materials may have consisted of straw and mud in the form of bricks for building construction. By way of comparison, advanced composite materials are routinely employed in modern aeronautic and spacecraft design, as well as other applications that require materials capable of performing in demanding conditions and harsh operating environments.
Engineered composite materials must typically be formed to conform to a particular shape. This generally involves manipulation of the reinforcement materials while controlling the matrix properties to achieve a melding event at or near the beginning of the component lifecycle. A variety of fabrication techniques may be employed in correspondence to specific design requirements. These fabrication methods are commonly termed ‘molding’ or ‘casting’ processes, as appropriate. Principle factors affecting the manufacturing methodology are the nature of the selected matrix and reinforcement materials. Another parameter is the total quantity of material to be produced.
Many commercially produced composites use a polymer matrix material often referred to as a resin or resin solution. The most common categories of these polymer materials are polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, as well as others. The reinforcement materials are often fibers, but ground minerals may also be used. Fibers may be woven into a textile material such as a felt, fabric, knit or stitched construction. Additionally, some composite materials employ an aggregate in place of, or in addition to, fibers.
Advanced composite materials generally use carbon fiber reinforcement and epoxy or polyimide matrix materials. These materials have found application as aerospace-grade composites, and typically involve laminate molding at high temperature and pressure to achieve high reinforcement volume fractions. Such advanced composite materials generally provide relatively high stiffness and/or strength:weight ratios. In terms of stress, the fibers generally serve to resist tension while the matrix serves to resist shear—and all materials present generally operate to resist compression.
Conventional bisorthodinitriles (often referred to as phthalonitrile monomers) polymerize to form relatively strong, high-temperature polyphthalocyanine thermosetting resins. Representative examples of bisorthodinitriles that are suitably adapted for producing such resins are disclosed in U.S. Pat. No. 4,056,560; U.S. Pat. No. 4,057,569; and U.S. Pat. No. 4,136,107—all to James R. Griffith and Jacques G. O'Rear.
Phthalonitrile monomers and phthalonitrile polymers of various types are generally described in U.S. Pat. Nos. 3,730,946; 3,763,210; 3,787,475; 3,869,499; 3,972,902; 4,209,458; 4,223,123; 4,226,801; 4,234,712; 4,238,601; 4,259,471; 4,304,896; 4,307,035; 4,315,093; 4,351,776; 4,408,035; 4,409,382; 4,410,676; 5,003,039; 5,003,078; 5,004,801; 5,132,396; 5,159,054; 5,202,414; 5,208,318; 5,237,045; 5,242,755; 5,247,060; 5,292,854; 5,304,625; 5,350,828; 5,352,760; 5,389,441; 5,464,926; 5,925,475; 5,965,268; 6,001,926; and 6,297,298.
The patents referenced vide supra generally teach methods for making and polymerizing phthalonitrile monomers. These monomers typically have two phthalonitrile groups, one at each end of a connecting spacer chain. The monomers may be cured, whereby the cross-linking occurs between cyano groups. These cross-linked networks typically have high thermal and oxidative stability.
Phthalonitrile monomers with aromatic ether oligomeric or polymeric spacer linkages are expected to be useful since they are predicted to have low melting points. Phthalonitrile monomers with a large window between the melting point and the cure temperature are generally desirable to control the rate of curing and the viscosity during the cure.
U.S. Pat. No. 4,259,471 to Keller et al. discloses a phthalonitrile monomer (often referred to as an oxyarylbisorthodinitrile) having a polyphenoxy spacer with from 1 to 10 phenyl groups in the spacer chain. The monomer is made by reacting 4-nitrophthalonitrile with an aromatic diol. The aromatic diol is a phenoxy chain with terminal hydroxy groups. Keller et al. also teaches that, when the polyphenoxy spacer contains one phenyl group, the monomer is the most difficult to cure and the phthalocyanine resin generated is the most rigid and brittle. Resins prepared from monomers with spacers containing 2 to 5 phenyl groups represent the best combination of economy and ease of preparation.
U.S. Pat. No. 6,756,470 to Keller et al. teaches that in bisphthalonitrile compounds containing polyphenoxy spacers, as the length of the polyphenoxy spacer moieties increases, the processing temperature of the phthalonitrile monomer is shifted to lower temperatures. The low melting point allows the monomer to have a lower viscosity at a given temperature than other phthalonitrile monomers. A low viscosity resin generally enables composite processing by resin transfer molding, resin infusion methods and filament winding without heating the curing mix to a temperature that initiates curing. Curing may be initiated when the mixture is in position and need not flow any further. Furthermore, a low melt viscosity and a larger processing window may be useful for fabrication of thick composite sections where the melt must impregnate thicker fiber preforms.
The curing mixture viscosity is a function of both the curing agent concentration and the melt temperature. Accordingly, low melting phthalonitrile monomers and curing agents that do not volatilize at elevated cure temperatures can enhance the processability of phthalonitrile-based composites. This may be desirable since most high temperature resins are not amenable to processing by cost effective methods such as resin transfer molding, resin infusion molding, filament winding and oven cure due to high initial viscosities, the evolution of volatiles during the cure, as well as solvent-related problems.
The generated thermoset has the advantage of desirable thermo-oxidative properties, which may be generally unaffected by the nature of the curing agent. The thermoset also has improved physical properties, such as toughness and processability, relative to systems with a short spacer between the terminal phthalonitrile moieties. Generally, toughness and brittleness are improved with lower cross-link densities. This may be achieved by using phthalonitrile monomers with longer spacer chains. The structural strength of the resins is comparable to that of epoxy and polyimide resins. These resins have many advantages over polyimides due to, for example, the absence of solvents, being less hydroscopic, not being thermoplastic with a low glass-transition temperature, and having better water resistance. U.S. Pat. No. 6,756,470 further teaches that the polyphenoxy spacer in the phthalonitrile should contain at least three phenyl groups